Prop Step Up

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TriangleTom
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Prop Step Up

Post by TriangleTom » Sun Jan 13, 2019 9:17 am

Is the use step up gearing for the propeller common in steam launches? I've only found reference to it being installed in Cyrene, listed in The Steam Launch, however I am unfamiliar with its implementation other than that a chain drive was used.

Given that our engines are typically operated at low RPM, and the difficulties in mounting a 20+ inch prop on a hull, it seems like there would be real advantages at the cost of added complexity.
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Lopez Mike
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Re: Prop Step Up

Post by Lopez Mike » Sun Jan 13, 2019 6:01 pm

Of course the bigger prop you can swing, the better.

If I was to do it I would look carefully at a toothed belt. Mostly for the silence. And corrosion resistance.

With V-belts it is sometimes necessary to use more than one belt to handle the torque and that gets you into buying sets of matched belts. Not they wear out that much but an added hassle. And they need good tension. Hard to arrange without extra prop shaft bearing and also I'm not sure what effect the loads would have on the engine main bearings. True of pretty much all reduction systems except a planetary gear box. It would be nice to resolve the side loads within the reduction apparatus. More complications.

Chain drive is very efficient but needs lubrication in a marine environment. I would be worrying about getting a rag or my finger into a sprocket. Maybe noisy?

There was one boat with a gearbox failure a few year ago in California. I don't know the details but they had to be towed in.

I would give up some protection against grounding to be able to swing a bigger prop. I have a rudder support strap under the prop on my new hull and it has a curve in it to clear the prop. Esthetically not great.
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Re: Prop Step Up

Post by barts » Sun Jan 13, 2019 9:21 pm

If I was to do it I would look carefully at a toothed belt. Mostly for the silence. And corrosion resistance.
For smaller engines this would work really well. For bigger ones, the size belt + pulleys needed becomes problematic
With V-belts it is sometimes necessary to use more than one belt to handle the torque and that gets you into buying sets of matched belts. Not they wear out that much but an added hassle. And they need good tension. Hard to arrange without extra prop shaft bearing and also I'm not sure what effect the loads would have on the engine main bearings. True of pretty much all reduction systems except a planetary gear box. It would be nice to resolve the side loads within the reduction apparatus. More complications.
Rainbow has a planetary reduction gear - 1:1.5 - because the 18"x26" prop is what fits in the bronze aperture , and the Burleigh 3x5x4 compound really wants 26 x 30+.
Chain drive is very efficient but needs lubrication in a marine environment. I would be worrying about getting a rag or my finger into a sprocket. Maybe noisy?
I'd fit a chain guard and keep a bit of oil in the bottom. Something like this, perhaps: https://www.uniguardmgc.com/oil-bath-un ... -sprocket/
There was one boat with a gearbox failure a few year ago in California. I don't know the details but they had to be towed in.
Navy K iirc. You need a serious gear box for an engine w/ a 7.75" LP.... if you hit the simpling valve at startup you've got over 2000 peak ft-lbs of effort on the gearbox. You don't need as a rating... but you need a generous overload capacity.

The best choice for efficiency is to pick an engine, boiler and boat such that you can spin the prop fast enough to use approx. 1:1 diameter/pitch ratio... but this is hard to do, and some people like slow turning engines. You can pitch it higher, but props with pitch 150% of diameter aren't easy to find. A gear step up lets one have a slow turning engine and a prop that fits under the boat. A smaller engine, spinnig a bit faster, is often the best choice from an engineering standpoint.

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Re: Prop Step Up

Post by DetroiTug » Mon Jan 14, 2019 12:18 am

I wouldn't become too preoccupied with a high pitch prop. My opinion to go along with the good ones above, but I think its better to let the engine spin with shorter cutoff with nearly the same steam consumption. Trying to get a lot of torque out of a slow turning engine looks nice but results in very high loading on the bearing surfaces. It can be compared to a 4 speed car and driving around in 4th gear all the time, the engine is not going to last very long. I have noticed boats that run "steam props" their engines need to be tightened up frequently. Let her spin. On the tug I run a 21X23 4 blade and it's about right. Decent hull speed and the engine seems happy.

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Re: Prop Step Up

Post by barts » Mon Jan 14, 2019 5:41 am

DetroiTug wrote:
Mon Jan 14, 2019 12:18 am
I wouldn't become too preoccupied with a high pitch prop. My opinion to go along with the good ones above, but I think its better to let the engine spin with shorter cutoff with nearly the same steam consumption. Trying to get a lot of torque out of a slow turning engine looks nice but results in very high loading on the bearing surfaces. It can be compared to a 4 speed car and driving around in 4th gear all the time, the engine is not going to last very long. I have noticed boats that run "steam props" their engines need to be tightened up frequently. Let her spin. On the tug I run a 21X23 4 blade and it's about right. Decent hull speed and the engine seems happy.

-Ron
Yup... if you halve the speed of the engine and still want the same power, you're going to need to have twice the torque - and twice the bearing loading. The main problem w/ running 500 rpm instead of 250 is sometimes one gets too much vibration/noise.

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Re: Prop Step Up

Post by Lopez Mike » Mon Jan 14, 2019 5:52 pm

It has been "Shake, Rattle and Roll" in my old hull above around 350 r.p.m. but the motor mounts were pretty much a trampoline. We'll see on the new hull.
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Re: Prop Step Up

Post by barts » Tue Jan 15, 2019 4:17 am

Lopez Mike wrote:
Mon Jan 14, 2019 5:52 pm
It has been "Shake, Rattle and Roll" in my old hull above around 350 r.p.m. but the motor mounts were pretty much a trampoline. We'll see on the new hull.
Otter has the same problems w/ her 2.5" x 2.5" single as the counterweights are too small; the old 2" x 2.5" single acting twin was much smoother at speed, of course. I went to a 16"x20" prop from her old 13" x 17" because the new engine ran away w/ the small prop, but at full throttle (180 psi) the crosshead and big end bearings really hate me.

- Bart
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Re: Prop Step Up

Post by malcolmd » Tue Jan 15, 2019 8:00 pm

There are at least 5 boats to my knowledge in the SBA, including mine :-)

They all use commercial toothed belt arrangements and work well..

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Re: Prop Step Up

Post by cyberbadger » Wed Jan 16, 2019 12:00 am

Nyitra is 6HP, and the belt she has now has not failed.

HTD(High Torque Drive) 30mm wide, 8mm pitch Gates Power Grip 3 belt.

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Re: Prop Step Up

Post by TriangleTom » Wed Jan 16, 2019 10:34 am

barts wrote:
Tue Jan 15, 2019 4:17 am
Lopez Mike wrote:
Mon Jan 14, 2019 5:52 pm
It has been "Shake, Rattle and Roll" in my old hull above around 350 r.p.m. but the motor mounts were pretty much a trampoline. We'll see on the new hull.
Otter has the same problems w/ her 2.5" x 2.5" single as the counterweights are too small; the old 2" x 2.5" single acting twin was much smoother at speed, of course. I went to a 16"x20" prop from her old 13" x 17" because the new engine ran away w/ the small prop, but at full throttle (180 psi) the crosshead and big end bearings really hate me.

- Bart
Bart-

I'm curious what you're doing for crosshead and big end lubrication that lets it stand up to that much pressure.
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